The transmission of electric power between one part turning at low speed and the other fixed, in general necessitates recourse to sliding contacts. This is particularly the case with satellites having alignable solar generators.
These sliding contacts involve numerous disadvantages such as: wear of the contacts, development of the opposing torque, electrical noise, and dissipation by Joule effect in the contacts. These disadvantages are very sharply amplified in the case of operation at very high vacuum.